1. Field of the Invention
The present invention relates to an apparatus and method for reducing banding artifacts during ink jet printing, in particular, by modulating the swath boundaries of a printhead using variable interlacing percentages.
2. Description of the Related Art
Thermal ink jet printers apply ink to a print medium such as paper by ejecting ink drops from one or more printheads through a column or array of nozzles. The printheads are mounted on a carriage assembly that is movable in a lateral direction across the print medium and ink drops are selectively ejected from the nozzles at corresponding ink drop placement locations. Specifically, each nozzle is associated with a heater resistor that generates heat when sufficient current passes through it, with the generated heat causing ink within an associated ink reservoir to be ejected from the nozzle. The printheads are moved in a series of lateral passes or swaths across the print medium, and between printing passes, the print medium is advanced in a longitudinal or advance direction. Since the printhead moves in a direction that is perpendicular to the advance direction of the print medium, in each pass, each nozzle passes in a linear manner over the print medium. The line associated with each nozzle which overlies the print medium is commonly referred to as a raster or raster line. A plurality of raster lines extending across the image area of the print medium are disposed vertically adjacent to each other in the advance direction.
A printer controller determines which resistors will be “fired” and the proper firing sequence so that a desired image is printed. However, ink drop placement errors can occur due to a number of factors, including incorrect timing or variable ink drop flight time. Further, if the amount that the print medium is advanced between printing passes is not precisely equal to a theoretical advance amount, either adjacent printed lines can overlap, or be visibly distant from one another. These effects of incorrect line spacing are referred to as line continuation errors.
Known methods exist for ameliorating the effects of ink drop placement errors. One such method is known as shingling or interlaced printing. For a two pass shingling mode (i.e., 50% interlace level), approximately 50% of the ink drops are placed on any given printhead pass. The candidate dots in a first pass are selectively addressed according to a checkerboard pattern, with the remaining dots printed on the next pass, after the print medium is advanced. For a printhead with vertically disposed colors, and a raster which contains cyan, magenta and yellow dots, printing at a 50% interlace level requires that the printhead be passed at least six times across each raster line. Employing a shingling method generally improves print quality, while at the same time reducing the printing speed as compared to printing without shingling.